1. Technical Field
The present invention relates to a lithium battery pack for use in a cordless power tool or the like.
2. Description of Related Art
Generally, a lithium battery includes a vanadium lithium battery, a manganese lithium battery and similar batteries, each of which uses a lithium aluminum alloy in the negative pole thereof and also uses organic electrolyte as electrolyte. Also, a lithium ion battery generally means a battery which uses cobalt oxide lithium in the positive pole thereof and graphite in the negative pole thereof and also uses organic electrolyte as electrolyte. In the present specification, for convenience sake, the organic electrolyte secondary battery, which includes the lithium battery and lithium ion battery, is generically called a lithium battery simply.
As the power supply of the cordless power tool, conventionally, there are often used a nickel-cadmium (N—Cd) battery, a nickel hydrogen (Ni-MH) battery and the like. However, recently, with the increasing need for the enhanced capacity and reduced weight of the power supply, there has been used the lithium battery.
The nominal voltage of an ordinary nickel-cadmium battery is 1.2 V, whereas the nominal voltage of a lithium battery is large, for example, 3.6 V, which means that the lithium battery has an advantage. On the other hand, the lithium battery is disadvantageous in that, when the lithium battery is overcharged or over-discharged, the lithium battery is greatly deteriorated. For example, when a lithium battery having a nominal voltage of 3.6 V is overcharged to or more than 4.5 V, the electrolyte thereof is decomposed, which causes the performance of the battery to deteriorate greatly. Also, when the lithium battery is over discharged to or less than 2.0 V, the performance of the battery is deteriorated greatly.
In view of the above problems, conventionally, there is provided a protection circuit which is used to monitor the overcharge or over-discharge of the respective battery cells contained within the battery pack; and, when the battery voltage of the respective battery cells becomes equal to or more than a first given value, and also when it becomes equal to or less than a second given value, the charge and discharge of the battery are stopped to thereby prevent the overcharge and over discharge of the battery.